1. Field of the Invention
The present invention is in the field of biological waste treatment in multiple stages utilizing periodic reversal of flow between the stages to secure improved efficiency.
2. Description of the Prior Art
Biological waste treatment systems rely on microorganisms to decompose organic materials. These microorganisms use the organic waste substances or substrates for growth and maintenance energy, thereby producing cell solids and various by-products and/or end products.
Generally, growth is considered to be related to the concentration of substrate by the Michaelis-Menten or Monod relationships as follows: ##EQU1## Combining equations (1) and (2) gives: ##EQU2##
This equation shows that if a substrate is fed to an accumulated active microorganism mass, M.sub.a, at a high concentration, S, the net rate of growth, R.sub.g, will be positive and the active mass will increase. At some specific substrate concentration where S=S.sub.m, the rate of growth will become zero. At substrate concentrations below S.sub.m, the microorganism population will shift into a decay mode, and net growth will be negative, that is, the active mass will begin to decrease.
Normally, biological waste treatment systems are designed for operation where net growth is positive. As a result, microorganisms grow and produce an excess accumulation that must be wasted, this excess being commonly referred to as sludge. Typically, substrate removal efficiency improves as the net growth approaches zero. If attempts are made in a single reactor system to operate at substrate concentrations in the negative growth range, the accumulated solids decay to the point that the system fails. However, it is desirable to operate as close to the point of zero growth as possible to improve the waste treatment efficiency and to minimize the net amount of excess sludge produced.
In Apr. 1982, Bandy and Messenger published a paper in which they sought to increase the capacity of a rotating biological contactor to treat shock loads by periodically reversing the direction of flow across two or more stages of such a contactor. The purpose was to increase the total inventory of biofilm within several stages. There was no disclosure of operating under the conditions under which the present invention is carried out.